1. Lenses
Converging and Diverging Lenses

2. Lens
A piece of glass that bends parallel rays of light so they cross and form an image.

3. Lens
Principal Axis: the line joining the centers of curvature of its surfaces.
Focal Length: the distance between the center of a lens and either focal point.

4. Converging (Convex) Lens
A lens that bends incoming parallel rays so they converge to a single point.
They are thicker in the middle than on the edges.

5. Converging (Convex) Lens
Converging Focal Point: the point at which a beam of parallel light converges.
Converging Focal Plane: the plane that passes through the focal point (perpendicular to the principal axis) where light can converge.

6. Diverging (Concave) Lens
A lens that bends incoming parallel rays so they diverge from a single point.
They are thinner in the middle than on the edges.

7. Diverging (Concave) Lens
Diverging Focal Point: the point from which the beam of light seems to come.
Diverging Focal Plane: a beam appears to come from a point on a focal plane.